Pi*R^2
3.142*4.242640687119285^2
56.55 (just round it to two past decimal)

So your port area using dual sixes would be 56.55 square inches. You can use math or just download WinISD Pro (http://www.linearteam.dk/default.aspx?download=winisdpro), which will yield you the same port area. Or as Immacomputer suggested just use PSPís online calculator (http://www.psp-inc.com/psp-inc.com/cgi-bin/port_length_calculator.cgi).